1. Field of the Invention
The present invention relates generally to a ceramic radiation shield and radiation detector using the same, and more particularly to a radiation detector used for computer-assisted tomography (CT) equipment relying on X rays, y rays or rays from other radiation sources, a ceramic radiation shield suitable for use in the same, and a method for making such a ceramic radiation shield.
2. Description of the Related Art
In radiographic CT equipment, arrays of radiation detectors are disposed at positions symmetrical to radiation sources (X-ray tubes, for example) with respect to a subject being examined, and the internal structure of the subject is observed by measuring the intensity of the radiation transmitted through the subject. Arrays of radiation detectors, which are equivalent to picture elements, should be manufactured into as small a size as possible, and arranged as densely as possible to improve resolution.
The radiation detector usually has a laminated construction of radiation scintillators and semiconductor photo-detector devices so that the scintillators which are open to the radiation-source side receive X-ray and other radiation beams. The scintillator, made of CdWO4, Bi4Ge3O12, Gd2O2S: Pr(Ce, F), emits visible light when exposed to incident radiation. The visible light enters on the semiconductor photo-detector element provided on the rear surface of the scintillator to convert it into electrical signals. If the radiation incident on the scintillator passes through the scintillator and enters again into adjacent scintillators, a phenomenon called crosstalk occurs, leading to lowered resolution. To cope with this, radiation shields made of Mo, W, Pb or other metallic sheets are provided between the scintillators to prevent radiation from transmitting through them.
Visible light, which is generated by the scintillators in the direction of the total solid angle, must be led to the semiconductor photo-detector elements provided on the rear surface of the scintillators. Consequently, the scintillator has a construction that its periphery, except the surfaces opposing the semiconductor photo-detector elements, is covered with a substance having good light reflectivity. The surfaces at which the scintillators adjoin each other are so large that white paint is filled in between the adjoining scintillator surfaces, or shields coated with white paint are interposed between the scintillators. As the white paint, a mixture of titanium oxide and a resin, such as epoxy, is often used.
For this reason, the radiation detector often has a construction that a radiation shield made of Mo, etc. which is bonded with a light reflecting sheet (or film), e. g. a resin titanium oxide on its both sides is interposed between the scintillators.
To make a radiation shield from Mo, W, Pb, etc., it is necessary to machine Mo, W, Pb or other metallic sheet into a thickness of 50 to 100 xcexcm. These metallic materials are extremely hard to machine into a desired thickness because they are softer in some cases, or harder in other cases, than iron, aluminum and others. This may often cause deviations in the dimensions of intervals between the scintillators.
In another manufacturing method, radiation shielding sheets made of Mo, W or Pb and scintillator sheets are laminated alternately, and the radiation receiving surface and the light emitting surface are lapped. During this lapping operation, the radiation shields tend to be left unpolished, protruding from the scintillators because the scintillators are easily polished away, while the radiation shields are hard to be polished away. Thus, when the laminate is mounted on a semiconductor photo-detector element, the protruded Mo, W or Pb could impair the photo-detector element surface.
When the light-reflecting sheet (or film) made of titanium oxide and a resin is used, other problems are likely to occur. In the manufacture of the light-reflecting sheet, the sheet is heated to about 80xc2x0 C. This heat has discolored the resin, such as epoxy, lowering the reflectivity of the reflecting sheet. Furthermore, the polishing solution or abrasive grains used in the manufacture has caused scratches on the surface, or contaminated the surface, leading to a local reduction in the reflectivity of the reflecting sheet.
Furthermore, uneven thickness or warpage has occurred during the manufacture of the light reflecting sheet, leading to changes in reflectance, or uneven output produced by the element coefficient.
It is therefore an object of the present invention to provide a radiation shield made of sintered ceramics that has high radiation shielding capability, is easy to machine, and can be used as a radiation shield in place of Mo, W and Pb.
It is another object of the present invention to provide a radiation shield made of sintered ceramics having light reflecting performance.
It is still another object of the present invention to provide a radiation detector incorporating radiation shields made of sintered ceramics.
It is a further object of the present invention to provide a manufacturing method of a radiation shield made of sintered ceramics.
Cross-sectional front views of a radiation detector according to the present invention are shown in FIGS. 1 and 2. In this radiation detector, a multitude of scintillators 2 are arranged adjacent to each other on a semiconductor photo-detector element 1, and radiation shields 3 are provided between the scintillators 2. In the present invention, the radiation shield 3 is made of a sintered ceramic material having radiation shielding capability. The radiation shielding rate of the sintered ceramics should be not lower than 90%, or more preferably not lower than 95%. In the radiation detector shown in FIG. 1, light reflecting films 4 are provided between the radiation shields 3 and the scintillators 2. As the light reflecting film 4, a previously known mixture of titanium oxide and epoxy resin may be used. The radiation shield 3 and the light-reflecting film 4 are collectively called as a separator. In the present invention, a mixture of 0 to 50 mol %, or more desirably 3 to 33 mol %, in total of at least one rare-earth oxide selected from the group of Gd, La, Ga, Y, Ce, Nd, Pr, Sm, Dy and Yb oxides, and 0 to 33 mol %, or more desirably 0 to 28 mol %, in total of at least one alkali-earth oxide selected from the group of Ca, Ba, Mg and Sr oxides, and the balance being at least one of oxides of V, Ta and Nb compose as the sintered ceramics for use as the radiation shield 3.
In the radiation detector shown in FIG. 2, the radiation shield 3 is made of a white sintered ceramics. Having not only radiation shielding capability but also high light reflecting capability, this sintered ceramic material has a radiation shielding rate of not less than 90%, or more desirably not less than 95%, and a light reflecting capability of not less than 70%, or more desirably not less than 90%. In this case, no separate light reflecting film is needed since the sintered ceramic material itself has light reflecting capability. In this case, therefore, only the radiation shield 3 is called the separator.
A mixture of 3 to 33 mol %, or more desirably 5 to 33 mol %, in total of at least one rare-earth oxide selected from the group of Gd, La, Ga and Y oxides, and 0 to 33%, or more desirably 0 to 28 mol %, in total of at least one alkali-earth oxide selected from the group of Ca, Ba, Mg and Sr oxides, and the balance being at least one of Ta and Nb, compose the white sintered ceramics according to the present invention.
If at least one of Ce, Nd, Pr, Sm and Yb oxides, or V oxides is used, the mixture does not result in a white ceramic material, but in a colored one. Thus, the resulting material has an effect of radiation shielding, but does not have a light reflecting effect due to reduced light reflectance.
In the present invention where the scintillators and the separators are bonded adjacent to each other, when a sintered ceramic material is used as the separator, the difference in thermal expansion coefficients between them should preferably be not more than 2xc3x9710xe2x88x926xc2x0 C., or more desirably not more than 1xc3x9710xe2x88x926/xc2x0 C.
FIG. 3 is a block diagram of the manufacturing process of sintered ceramics, which is obtained by weighing and wet blending rare-earth, alkali-earth oxides and V, Nb and Ta oxides so as to achieve the aforementioned composition. The mixture is then calcined to approx. 1000xc2x0 C. in air for an hour. The calcined mixture is ground to a grain size of about 0.8xcx9c1.0 xcexcm, and a binder (PVA) is added to the ground mixture for granulation. The granulated grains are formed into a green body in a press, sintered in the air at a temperature of 1600 to 1800xc2x0 C., and subjected to HIP (hot isostatic pressing) at 1400 to 1800xc2x0 C., or more preferably at 1500xc2x0 C. and 1000 atmospheric pressure, to increase the density of the sintered body. After that, the body is annealed in a 1000 to 1300xc2x0 C. air or oxygen as a measure to adjust the amount of oxygen. In place of sintering and HIP treatments, the mixture can be hot-pressed at 1400 to 1800xc2x0 C.